1. Field of the Invention
The present invention relates to a control method of an uninterruptible power supply (UPS), and more particularly to a control method of a UPS capable of extending a discharge time under a no-load condition.
2. Description of the Related Art
Uninterruptible power supplies (UPS) are the power backup equipment serving to supply backup power to a load when the AC mains is abnormal, such as power outage, overvoltage/undervoltage, occurrence of surge current and the like, so as to continuously supply an operating power to the load and prevent some mission-critical equipment, such as computers, telecommunication networks, private branch exchanges (PBX) and the like, from losing data or getting out of control.
Conventional UPSs can be generally classified as on-line UPSs, off-line UPSs and line-interactive UPSs.
The On-line UPSs serve to separate the AC mains and load so that the mains power is sent to the UPSs and is converted into DC power to charge a battery set instead of being directly supplied to the load. Meanwhile, the DC power is further converted into AC power, and then the AC power is supplied to the load. Once power outage or power abnormality occurs, a battery mode is enabled and AC power is converted from the DC power of the battery set and is supplied to the load. Basically, the power waveforms outputted by the on-line UPSs and by the AC mains both pertain to sinusoidal waves.
The off-line UPSs usually play a power backup role. When the AC mains is normal, the mains power is directly supplied to the load for simultaneously charging a battery set. Once the AC mains fails, a mains power supply loop of the off-line UPSs is automatically disconnected from the AC mains and a battery mode is enabled so that the AC power converted from the DC power of the battery set is further supplied to the load. As the waveform of the AC power converted from the DC power of the battery set pertains to a square wave, the converted AC power can be supplied to capacitive loads only.
The line-interactive UPSs are equipped with a voltage fluctuation circuit, are operated the same way as the off-line UPSs, and do not intervene to supply power throughout the entire course. Instead, the line-interactive UPSs monitor the condition of the mains supply on a real-time basis, and instantly perform voltage fluctuation or enter a battery mode to replace the AC mains and continuously supply power to the load when the AC mains supply is abnormal.
The above UPSs enter a battery mode and keep converting the DC power of the battery set into AC power and supplying the AC power to the load when the AC mains fails. However, even during a no-load condition or when the load turns off, upon entering the battery mode, the UPSs continuously discharge. When a power capacity of the battery set is discharged down to a low voltage level, the UPSs automatically shut off. To ensure that the UPSs can immediately supply power to the load having a power demand and that normal power supply is not affected by draining the power of the battery set when the load turns off, the discharged power of the battery set should be kept at minimum during the no-load condition. In other words, the discharge time of the battery set under the no-load condition should last as long as possible so as to be responsive to a sudden power demand.